General processes by which geothermal fluids can be used to generate electric power are known and have been known for some time. Naturally pressurized geothermal brine having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence.
Although, as above mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of highly saline geothermal brine, have often been encountered in practice. These problems have frequently been so great as to prevent the production of electric power at competitive rates and, as a consequence, have greatly impeded the progress of flashed geothermal brine power plant development in many areas.
These severe problems are caused primarily by the complex composition of geothermal brines. At natural aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of from 400 to 500 psig, the brine leaches large amounts of salts, minerals and elements from the aquifer formation. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron and cadmium. In addition, many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As the natural brine pressure and temperature are substantially reduced in power plant steam production (flashing) stages, chemical equilibrium of the brine is disturbed and saturation levels of impurities in the brine are typically exceeded. This causes impurities and silica to precipitate from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Assuming, as is common, that the brine is supersaturated with silica at the wellhead, in high temperature portions of the brine handling system, for example, in the high pressure brine flashing vessels, heavy metal sulfide and silicate scaling typically predominates. In lower temperature portions of the system, for example, in atmospheric flashing vessels, amorphous silica and hydrated ferric oxide scaling has been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing scale rates, extensive facility down time for descaling operations may be required, unless scale reducing processes are used. Associated injection wells may also require frequent and extensive rework and new injection wells may, from time to time, have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed towards developing effective processes for eliminating, or at least very substantially reducing, silica scaling in flashed geothermal brine handling systems. One such scale reduction process disclosed in U.S Pat. No. 4,370,858 to Awerbuck, et al, involves the induced precipitation of scale-forming materials, notably silica, from the brine in the flashing stage by contacting the flashed brine with silica or silica-rich seed crystals. When the amount of silica which can remain in the brine is exceeded by the brine being flashed to a reduced pressure, silica leaving solution in the brine deposits onto the seed crystals. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much larger surface area than the exposed surfaces of the flashing vessels, but also the silica from the brine tends, for chemical reasons, to preferentially deposit onto the seed crystals. Substantially all of the silica precipitating from the brine precipitates onto the seed crystals instead of precipitating as scale onto vessel and equipment walls.
To protect the injection wells from plugging by the substantial quantity of precipitate contained in the brine, it is customary to provide a brine clarification stage. Generally the brine clarification stage comprises at least one clarifier vessel into which the brine is introduced to permit the precipitate to settle therefrom. A clarified brine overflow is introduced into a filtering stage and then subsequently introduced into the reinjection well. The precipitate (sludge) from the clarifier is removed and dewatered, generally through the use of a filter press. The amount of such sludge requiring disposal is substantial. For example, a 50 megawatt power plant which requires a brine flow rate of about 5 million pounds an hour will produce approximately 30 tons of sludge per day.
A typical filter press comprises an alternate assembly of plates covered on both sides with a filter medium, usually a cloth, and hollow frames that provide space for cake accumulation during filtration. Generally the frames have feed and wash manifold ports, and the plates have filtrate drainage ports. The plates and frames are pressed together during filtration to form a water tight closure between two end plates, one of which is stationary. The press may be closed manually, hydraulically or by a motor drive. A variety of feed and filtrate discharge arrangements are available. The filter press has the advantage of simplicity, low capital cost and flexibility. The filter capacity is readily varied by adding or removing plates and frames. A disadvantage of the filter press is the relatively short filter-cloth life due to the mechanical wear of emptying and cleaning the press (often involving scraping the cloth) and high labor costs.
During the silica crystallization process, many other materials precipitate from the brine onto the seed material along with the silica. Thus, although the sludge referred to above comprises mostly silica, it will also contain significant amounts of other materials such as barite, arsenic and heavy metals including lead, copper and zinc which, above specific levels of concentration, may be considered as toxic and require disposal in a toxic waste dump. It will be appreciated that the cost of disposal of such large amounts of sludge in a toxic disposal site would be substantial. Further, the cost can be expected to increase as larger geothermal brine power plants are constructed (with the attendant increase in the quantity of sludge produced), as allowable concentrations of heavy metals in the sludge are reduced by governmental regulations and as the number of toxic waste dumps decrease or become more remotely located. Even if the sludge is non-toxic, the sheer volume involved makes its disposal costly.
It has been suggested to use the geothermal sludge to make a concrete-like material which can be used for construction purposes. The sludge contained in a filter press is washed, and the washed sludge is combined with a portland type cement and an activating media to produce a structural concrete material. A condensate of steam, derived from the geothermal brine being processed, may be used as a wash water for the sludge contained in the filter press. It has been found that the cloth, comprising the filter media of the filter press, requires replacement at about one week intervals at a substantial cost.
It is an object of the invention to provide a method of treating a steam condensate derived from geothermal brine to enhance its utility as a wash water for a brine filter.
It is another object of the invention to provide a means for extending the life of a filter media used for removing water from a brine precipitate.